Wrap context_args to avoid splatting hashes

As @urbanautomaton points out on #478, because hashes support `to_a` they get splatted when passed as `context_args` [here](https://github.com/drapergem/draper/blob/668661f13aa2d216ea7ef3c88b4e11da102086e5/lib/draper/factory.rb#L83). This patch fixes this bug by wrapping the args in an array before splatting them.